A typical four-stoke internal combustion engine involves four piston strokes. The first stroke is the intake or suction stroke in which the piston moves down the cylinder and creates a partial vacuum in the cylinder. Intake air alone, or a mixture of air and fuel, is forced though the inlet valve into the cylinder by atmospheric pressure which is now greater than the pressure in the cylinder. During this stroke, the exhaust valve stays closed. The second stroke is the compression stroke wherein the piston moves up in the cylinder with both valves closed. The intake air or air and fuel mixture is compressed and the pressure inside the cylinder rises. The third stroke is the power stroke. Near the end of the compression stroke, the air and fuel mixture is ignited by an electric spark from the spark plug in the case of a spark-ignition engine. In a compression-ignition engine, fuel directly injected into the cylinder mixes with the compressed air and ignites spontaneously. The combustion that results causes the temperature within the cylinder to rise and creates enough pressure to force the piston down again. On the fourth stroke, or exhaust stroke, the piston moves up again and forces the burned gases out of the cylinder and into the exhaust system. This cycle repeats itself the entire time the engine is running.
In an internal combustion engine, the engine oil is contained in the sump or crankcase. The oil pump draws the oil from the crankcase, forces it through a filter and then circulates the oil through passages drilled in the engine block to bearings, the valve mechanism, and the pistons and cylinder walls before returning to the crankcase. During its circulation through the engine, the oil serves to lubricate parts, keep the engine free of rust and deposits, cool hot internal surfaces, and seal the piston rings and valves against leakage of combustion gases.
In any engine oil, the detergent-dispersant, anti-wear, oxidation, and rust and corrosion additives eventually become depleted and the oil loses much of its ability to prevent deposits from accumulating on critical engine parts. Among the critical engine components affected by improperly maintained oil are the piston rings and the piston itself.
Piston rings are typically spring-loaded for a tight fit against walls of the engine's cylinders; the compression rings prevent the escape of combustion gases into the crankcase and help maintain combustion chamber pressure while the oil control ring restricts oil movement from the crankcase past the piston body. Hard, baked-on deposits formed from the contaminants in the oil may cause the rings to stick and lose their sealing ability, thereby allowing the oil to leak past the rings into the combustion chamber where it is burned. Combustion of the engine's oil in this manner results in rapid oil deterioration, increased oil consumption, and is a source of contaminants that can reduce the effectiveness of the car's emission control devices. Additionally, stuck piston rings cause a loss of power and allow combustion products (“blow-by”) to enter the crankcase, further contaminating the oil and accelerating its deterioration.
In addition to contaminants in the oil, recent studies of the oil film thickness of the piston rings in certain reciprocating compressors (see IMechE 2001 C591/013/20021) have suggested that a major factor negatively affecting oil consumption is insufficient or inadequate oil drainage by the piston itself. The accumulation of excessive hydrodynamic pressure at the piston rings can force the rings into the grooves in an undesirable manner, thereby permitting an excessive amount of oil to pass through the rings' running faces. This excess oil is then forced into the combustion chamber where it is burned. Thus, there is a need for a piston that effectively drains the oil needed to lubricate the piston within the cylinder away from the piston rings and back to the crankcase portion of the engine or reciprocating compressor. Such a piston should significantly reduce the pressure build-up at the piston rings, thereby extending the life of the oil, piston rings, and engine or compressor.